add gradient scrollbar styling

[saurabhhhcodes]

What changed

• Added gradient-styled custom scrollbars across the app.
• Applied a dark track with a purple → pink → orange thumb to match the auth and chat visual language.
• Kept the scrollbars thin and visible for accessibility.

Why

The default browser scrollbar felt out of place next to the app's neon/glassmorphism styling.

Impact

The interface feels more cohesive without changing layout or app behavior.

Validation

• git diff --check
• npm run build in frontend

Closes #176

[vercel]

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